Are you sure the surface is in the data (meaning mapped)?
If the surface is in OSM (not many roads have this), you can display it on the map as well as in details of the calculated route. But just an example in my town, from random route:
distance 4,6 km, surface asphalt 2,33 km, unknown 2,41 km
another one:
distance 8 km, surface ground 1,81 km, asphalt 829 m, unknown 5 km
in both cases, "unknown" would be asphalt, it is just not mapped (it is taken as default for the category of the road)